The compound 2,6-diaminopyridine (“DAP”), which is shown by the structural formula below
is a useful starting material for preparing monomers for rigid rod polymers such as described in WO 94/25506, as well as for dyes, metal ligands, medicines and pesticides.
It is well-known to prepare DAP by means of the Chichibabin amination reaction in which pyridine is reacted with sodium amide in an organic solvent. This is a complicated reaction requiring relatively severe conditions (e.g. 200° C. at elevated pressure). Additionally, handling sodium amide and isolating the desired product from this complex mixture are difficult operations to perform on a commercial scale.
Elvidge and coworkers prepared glutarimidine (piperidine-2,6-diimine, “GI”) and several glutarimidine derivatives, as described in J. Chem. Soc., 1959, 208-15. The authors attempted to dehydrogenate 2,6-diphenyliminopiperidine, α-phenylglutarimidine, and 3-phenyl-2,6-diphenyliminopiperidine by distillation, by heating them in solvents with palladium/charcoal, and by treatment with chloranil or tetrachloro-o-benzoquinone. They apparently found no evidence for the formation of substituted diaminopyridines, and it appears that they concluded that the glutarimidines showed no great readiness to undergo dehydrogenation, and that no evidence for the formation of substituted diaminopyridines was obtained in preliminary attempts to dehydrogenate.
A need thus remains for an improved process for the formation of derivatives of pyridine.